The exemplary embodiments described herein relate to nuclear magnetic resonance (NMR) logging for determining the porosity and the pore type of subterranean formations.
When drilling a wellbore penetrating a subterranean formation, the drilling fluid (or drilling mud) infiltrates into the formation surrounding the wellbore. This infiltration occurs over multiple steps. Initially, as the drill bit exposes the formation, the drilling mud, including solid particles like weighting agents and drill cuttings, migrates into the pore spaces of the formation that is close to the wellbore wall. During this step of drilling mud infiltration, the solids form an internal mud cake in the formation at the wellbore by bridging the pores in the formation. As the internal mud cake forms, the fluid in the drilling mud readily flows into the formation, which is sometimes referred to as spurt invasion. Then, after the internal mud cake forms and pore bridging stabilizers, an external mud cake starts to build up on the wellbore wall. As the external mud cake begins to form, the fluid in the drilling mud infiltrates into the surrounding formation to a lesser degree by a mechanism known as transient invasion or static invasion. As the external mud cake thickens, infiltration of the fluid of drilling mud into the surrounding formation decreases until a steady state is reached. Even though it can take hours to build up an external mud cake of 2.5 mm, the most significant amount of fluid infiltration from the drilling mud into the surrounding formation is believed to occur within a few seconds.
NMR tools are often used to analyze the properties of the formation close to the wellbore such as porosity and pore type. Generally, the NMR tool produces a radiofrequency (RF) excitation pulse that penetrates into the formation and interacts with the fluids therein. This interaction then produces an NMR signal that is measured by the NMR tool. Depending on the RF excitation pulse and NMR signal detection parameters, various NMR properties may be measured. Because NMR measurements interrogate the composition of the fluid that interacts with the RF excitation pulse, the amount of fluid infiltration from drilling muds can affect NMR measurements and the formation properties derived. For logging-while-drilling (LWD) NMR measurements, the fluid composition in the portion of the formation being interrogated is changing due to the spurt invasion and transient invasion, which increases the error associated with the NMR measurements and the formation properties derived therefrom.